linux-kernel - [PATCH] iommu/qcom: Replace zero-length array with flexible-array member

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Message-ID: <20200213001401.GA28587@embeddedor.com>
Date:   Wed, 12 Feb 2020 18:14:01 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Rob Clark <robdclark@...il.com>, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Joerg Roedel <joro@...tes.org>
Cc:     iommu@...ts.linux-foundation.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] iommu/qcom: Replace zero-length array with flexible-array
 member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
---
 drivers/iommu/qcom_iommu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iommu/qcom_iommu.c b/drivers/iommu/qcom_iommu.c
index 39759db4f003..f1e175ca5e4a 100644
--- a/drivers/iommu/qcom_iommu.c
+++ b/drivers/iommu/qcom_iommu.c
@@ -48,7 +48,7 @@ struct qcom_iommu_dev {
 	void __iomem		*local_base;
 	u32			 sec_id;
 	u8			 num_ctxs;
-	struct qcom_iommu_ctx	*ctxs[0];   /* indexed by asid-1 */
+	struct qcom_iommu_ctx	*ctxs[];   /* indexed by asid-1 */
 };
 
 struct qcom_iommu_ctx {
-- 
2.23.0